1. Field of the Invention
The present invention relates to an optical transmission system, and a pumping light source stopping device and method to be used in same system, and more particularly to the optical transmission system being suitably used to ensure safety from dangerous highly output light being radiated at time of attachment and detachment of connectors, breakage of optical fibers, or a like, and a pumping light source stopping device and method to be used in same system.
The present application claims priority of Japanese Patent Application No. 2004-184802 filed on Jun. 23, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, due to widespread use of the Internet or a like, amounts of information required to be transmitted have increased greatly. In a WDM (Wavelength Division Multiplexing) transmission system out of such optical transmission systems as used in the above cases, in order to transmit a plurality of pieces of data, by utilizing a property that light waves having different wavelengths do not interfere with each other, a multiplexed signal light obtained by multiplexing a plurality of optical data signal waves, each carried by a corresponding one of carrier light waves each having a different wavelength, is transmitted through one optical fiber cable. By doing so, it is made possible to dramatically increase amounts of information that can be transmitted by one optical fiber cable when compared with a case in which each piece of data is transmitted by an individual optical fiber cable. In the optical fiber cable between a signal transmitting device and a signal receiving device used in the WDM transmission system, the multiplexed signal light is attenuated and, therefore, amplification and correction of the multiplexed signal light is required. As a result, conventionally, an optical fiber amplifier contains a trace element such as Erbium or a like. However, since there is a limit to a wavelength band in which an optical signal can be amplified and to a light transmission capacity, use of a Raman amplifier increases in recent years. The Raman amplifier, by using Raman scattering (physical phenomenon), has a characteristic to amplify signal light having a wavelength being longer by 100 nm than that of pumping light and, therefore, the effect of amplifying signal light in almost all the wavelength bands can be obtained.
The conventional WDM transmission system, as shown in FIG. 3, is so configured that optical signal transmitters 11, 12, . . . , 180, an optical multiplexer 2, an optical relay transmission path (wherein a relay device is included) 3, an optical demultiplexer 4, and optical signal receivers 51, 52, . . . , 580 are connected by an optical fiber. The optical signal transmitters 11, 12, . . . , 180 receive input electrical signals i1, i2, . . . , i80, respectively, and modulate eighty channels of carrier light waves having different wavelengths by using the input electrical signals i1, i2, . . . , i80, respectively to generate optical signals A1, A2, . . . , A80 to be transmitted over (via) eighty channels. The optical multiplexer 2 generates a multiplexed signal light “b” in a form of one light bundle which is obtained by wavelength-division- multiplexing of light signals A1, A2, . . . , A80 transmitted over (via) eighty channels into a wavelength range between about 1574 nm and about 1609 nm. The multiplexed signal light “b” is transmitted through the optical relay transmission path 3. The optical demulitiplexer 4 generates, by demultiplexing the multiplexed signal light “b” received from the optical relay transmission path 3, optical signals C1, C2, . . . , C80 to be received via eighty channels. The optical signal receiver 51, 52, . . . , 580 receive the optical signals C1, C2, . . . , C80 to be received and demodulate electrical signals D1, D2, . . . , D80 to be output via eighty channels.
FIG. 4 is a diagram showing configurations of the optical relay transmission path 3 shown in FIG. 3. The optical relay transmission path 3, as shown in FIG. 4, includes an optical amplifier 11, an optical fiber 12, a branching coupler (CPL) 13, a signal light removing filter (SwPF) 14, a pumping light monitor 15, an optical multiplexing/demultiplexing unit (WDM) 16, a pumping light source 17, a controller (COUNT) 18, a controller (COUNT) 18, and an optical amplifier 19.
The optical amplifier 11 amplifies a multiplexed signal light “b” in a manner to obtain a specified gain. The optical fiber 12 is made up of a dispersion shift fiber having a length of about 80 km and has an optical loss being about 20 dB, which is used to transmit the multiplexed signal light “b” obtained by amplification in the optical amplifier 11. The branching coupler (CPL) 13 is an optical fiber fusion type passive component which causes pumping light transmitted through the optical fiber 12 to branch into pumping light “c” to be transmitted to the optical multiplexing/demultiplexing unit 16 and into branched light “d” to be transmitted to the signal light removing filter 14 at a rate. of 95% and 5% respectively. The signal light removing filter 14 is a micro-optics type optical passive component, which removes signal light from the branched light and outputs pumping light “e”. The pumping light monitor 15 is made up of photodiodes and detects the pumping light “e”. The pumping light source 17 is made up of parts including semiconductor laser diodes and emits pumping light “f” for distributed Raman amplification of about 1 W in a wavelength band of 1.48 μm.
The optical multiplexing/demultiplexing unit 16 is a micro-optics type optical passive component which outputs multiplexed pumping light “g” obtained by multiplexing the pumping light “c” to be output from the branching coupler 13 and the pumping light “f” to be output from the pumping light source 17. The optical amplifier 19 amplifies the multiplexed pumping light “g” in a manner to obtain a specified gain. The controller 18 makes an operation of the pumping light source 17 stop instantaneously when the pumping light “e” having a specified value is detected by the pumping light monitor 15. For example, when an optical connector (not shown) used in relaying for the optical fiber 12 is broken due to maintenance work or a like, the pumping light “e” having a transmitting energy level of 1 W (=+30 dBm) is radiated from the above optical connector. This light radiation is dangerous for a maintenance worker and, therefore, it is desirous to instantaneously stop operations of the pumping light source 17. When the optical connector is disconnected, Fresnel reflection with a return loss of about 14 dB occurs at its end, the pumping light “e” of about +16 dBm returns back to the optical fiber 12. At this time point, since the pumping light “e” is detected by the pumping light monitor 15, operations of the pumping light source 17 is instantaneously stopped.
In addition to the above WDM transmission system, various conventional technologies of this-type are disclosed in, for example, Japanese Patent Application Laid-open Nos. 2002-182253 (patent reference 1), 2003-032192 (patent reference 2), 2003-218796 (patent reference 3), 2003-264509 (patent reference 4), and a like.
In the optical transmission system disclosed in the patent reference 1 (Abstract, FIG. 1), pumping light is supplied by a pumping light supplying means to an optical fiber and signal light traveling through the optical fiber is Raman-amplified. Power of Raman-amplified signal light is detected by a signal light power detecting means and whether or not supply of pumping light to the optical fiber should be stopped is judged by a judging means according to the power of signal light. If it is judged that the supply of pumping light should be stopped, the supply of the pumping light to the optical fiber is stopped. This can prevent the pumping light for Raman amplification from leaking to the outside and enables reduction of a danger caused by the leaked pumping light.
In the optical transmission system disclosed in the patent reference 2 (Abstract, FIG. 1), a terminal device transmits a monitoring signal made up of monitoring instruction signals and response carrier signals. Each of the monitoring instruction signals and response carrier signals has a different light wavelength. The relay device configured to perform light amplification using an optical fiber as an optical fiber transmission line generates a response signal obtained by superimposing response information on a response carrier and then transmits the response signal to the terminal device. Due to this, even if a line failure occurs in the vicinity of the relay device, it is made possible to exercise monitoring control, which improves reliability.
In the optical transmission system disclosed in the patent reference 3 (Abstract, FIG. 1), when a light signal loss in an optical fiber is detected by a light signal discontinuation detecting means, pumping light being supplied from a pumping light source is interrupted by a pumping light interrupting means and, therefore, even in a system using distributed Raman amplification, pumping light is interrupted automatically and rapidly at time of breakage of a fiber and of attachment and detachment of a connector.
In an optical communication module disclosed in the patent reference 4 (page 8, FIG. 2), signal light is transmitted from a signal transmitter with monitoring light being added thereto and, when monitoring light (light for detecting fiber bending) comes not to be detected by a signal receiver at time of breakage of a fiber and of attachment and detachment of a connector, supply of pumping light is stopped. A wavelength of the monitoring light is set at a U band (for example, 1625 nm to 1675 nm) while a wavelength of signal light is set at a C band (for example, 1530 nm to 1565 nm), or L band (for example, 1625 nm to 1675 nm). Moreover, even if a light interrupting device fails, a maintenance worker is made to recognize leakage of light by using visible rays. Even if bending of an optical fiber occurs, a highly sensitive function of interrupting light can be achieved. This enables safety of operations of a high-output optical fiber communication system to be maintained and needless consumption of energy to be suppressed.
However, such conventional optical transmission systems as described above have problems. That is, in the conventional WDM transmission system shown in FIG. 4, even when the optical connector is connected therein, small reflection with a return loss of about 25 dB occurs and, therefore, when pumping light of about +5 dBm has already returned back to the optical fiber 12, it is necessary to set the controller so as not to stop operations of the pumping light source 17. Here, if breakage (shown by the X mark in FIG. 4) of the optical fiber 12 occurs at a distance of a transmission attenuation of 10 dB or so from a Raman light source, pumping light having a transmitting energy level of 1 W (=+30 dBm) incurs a loss of 10 dB and pumping light of +20 dBm is radiated from the place where the breakage of the optical fiber 12 has occurred. Such radiation is dangerous and, therefore, it is desirous to instantaneously stop the pumping light source 17. When the optical fiber 12 is broken, Fresnel reflection occurs at an end of the optical fiber 12 and, therefore, light incurs a loss of 20 dB which includes losses occurring in both going and returning courses between the end of the optical fiber and the position where the breakage has occurred and, as a result, pumping light of about −4 dBm returns-back to the Raman light source. Since the energy level of −4 dBm is smaller than that of about +5 dBm, operations of the Raman light source (pumping light source 17) do not stop. As described above, the conventional WDM transmission system has the problem in that, since the breakage of the optical fiber 12 or a like is detected by the pumping light reflected at an end of the broken portion, operations of the pumping light source 17 do not stop instantaneously and dangerous high output light is radiated from the end face.
Moreover, the conventional optical transmission system disclosed in the patent reference 1, since it is so configured that power of Raman-amplified signal light is detected by a signal light power detecting means, has the same problem as in the WDM transmission system shown in FIG. 3. Also, the conventional optical transmission system disclosed in the patent reference 2, since it is so configured that a monitoring signal made up of monitoring instruction signals and response carrier signals each having a different optical wavelength is transmitted, has the problem in that configurations of the optical transmission system become complicated. Also, the conventional optical transmission system disclosed in the patent reference 3, since it is so configured that a loss in an optical signal in the optical fiber is detected by the light signal discontinuation detecting means, has the same problems as in the WDM transmission system shown in FIG. 3. Furthermore, in the optical communication module disclosed in the patent reference 4, the object of the invention is to take countermeasures against bending of optical fibers, which is different from that of the present invention.